The basic technology involved in the formation of microcapsules has been known for some time. In connection with the present disclosure the term "microcapsule" will encompass microspheres. In some literature the term "microcapsule" is intended only to encompass hollow spheres whereas "microspheres" is the term used to encompass solid spheres. In connection with the present invention the microcapsules are spherically shaped particles (solid or hollow) comprised of a polymeric material and have a diameter of 200 microns or less.
Basically, microcapsules are produced by dispersing a core material into a nonsolvating fluid, shearing the fluid to control the particle size of the core material and then inducing an encapsulating polymer to phase separate at the interface between the nonsolvating fluid and the core material. In general terms, the polymer or polymer precursors which are to be the encapsulating material may be initially dispersed along with the core material, or may be initially dispersed in the nonsolvating fluid, or may be added to one or both phases during the course of the encapsulating. A variety of physical or chemical methods may be employed to induce phase separation during the wall formation step including changing the temperature, pH, type or amount of solvent, inducing polymerization or coacervation processes or reactions and the like. One of the simplest methods of forming microcapsules is the solvent evaporation process. In this process, a polymer is first dissolved in a volatile organic solvent that is immiscible in water. Methylene chloride is a preferred solvent due to its high volatility and ability to act as a solvent with respect to a wide range of polymers. Other useful solvents include chloroform, carbon tetrachloride and ethyl acetate. It should be noted that many of the solvents suitable for use in connection with microcapsule formation do have a finite degree of water solubility even though they are normally classified as water-insoluble solvents. Further, it should be noted that a mixture of solvents can be used.
Once a desired coating polymer is dissolved in a casting solvent an active ingredient can be added to the solution. The active ingredient is any component which is to be encapsulated in the microcapsule. Throughout this specification the term "active ingredient" is at times abbreviated as A.I. The active ingredient may be dissolved in the polymer solution or may be completely insoluble in the solution and form a dispersion.
When the active ingredient is substantially insoluble in the polymer solution the active ingredient should be finely milled so that the mean diameter of the particle sizes of the active ingredient is sufficiently small relative to the desired mean diameter of the microcapsules. In connection with the present invention if the active ingredient is insoluble in the polymer solution the active ingredient should be milled so that the mean diameter is substantially below 200 microns, preferably below 100 microns and more preferably below 50 microns. Disclosures such as in U.S. Pat. No. 4,558,690, issued Dec. 17, 1985, refer to "microspheres" and "microcapsules" having an average diameter of 200-800 microns.
The polymer/active ingredient/solvent mixture is often referred to as the "oil phase." The oil phase is emulsified in water to form an oil-in-water emulsion. The size of the oil phase droplets obtained is determined by the type and amount of surfactant and the degree of agitation during the emulsification step. The size of the oil phase droplets determines the size of the microcapsules produced by the process. The emulsification or mixing together of the oil phase and the water phase can be carried out using different types of equipment such as high speed blenders in order to produce smaller microcapsules or different types of agitators for producing larger microcapsules. In order to facilitate the emulsification of the oil phase within the water phase a macromolecular surfactant is normally dissolved in the water phase before the oil phase is added. Dispersing agents (surfactants) which are commonly used in connection with this technology include partially hydrolyzed (88%) poly(vinylalcohol) (PVA), polyethylene oxide and propylene oxide block copolymers, polyacrylic acid and the like.
After the desired oil droplet size has been obtained, the system is stirred at a constant rate and the solvent evaporates. The evaporation can be facilitated by a variety of technologies known to those skilled in the art such as the use of a closed reduced pressure and a range of evaporation temperatures can be used. Once the solvent evaporation appears to be complete the capsules are separated from the suspending medium by filtration and thereafter are washed and dried. The maximum drying temperature must of course be such that it does not damage the microcapsules or cause them to fuse together.
It was recognized relatively early that it was possible to produce microcapsules which could be ruptured upon exposure to heat. Such microcapsules are disclosed within U.S. Pat. No. 3,317,433 issued May 2, 1967. These microcapsules often incorporated a substance which upon heating produced a gas which ruptured the microcapsule. The microcapsules often included a liquid or heat-liquefiable material as the active ingredient which material was generally combined with another material either in other microcapsules or present on a substrate in order to produce colors or markings on the substrate.
A considerable amount of literature now exists with respect to the use of microcapsules in connection with producing heat-sensitive recording materials. One such material is disclosed within U.S. Pat. No. 4,682,194 issued Jul. 21, 1987. The microcapsules present on the heat-sensitive recording material include a dye and are comprised of a polymer which has a glass transition point (T.sub.g) in the range of from about 60.degree. C. to 200.degree. C. By heating the microcapsules the polymeric material making up the microcapsules is transformed from a "glassy state" to a "rubbery state" which permits the active ingredient present within the microcapsule to permeate the wall of the microcapsule.
A related disclosure describing microcapsules used in connection with heat-sensitive recording materials is given within U.S. Pat. No. 4,742,043 issued May 3, 1988. In a similar manner the microcapsules are comprised of polymeric materials which have a glass transition point (T.sub.g) which is capable of preventing the active ingredient from leaving the capsule at lower temperatures but which quickly allows the active ingredient to exit from the microcapsule at higher temperatures.
Another heat-sensitive recording material is disclosed within U.S. Pat. No. 4,749,679 issued Jun. 7, 1988. Like the patents discussed above the microcapsules include an active ingredient which is capable of forming a color when it contacts an active ingredient present on the substrate or present within other microcapsules. The heat-sensitive recording material provides a heat-sensitive layer of microcapsules which contain a color former or a developer which when they contact each other upon heating form a color. The layer contains a plasticizer for the wall of the microcapsule and a compound which has an effect on depressing the melting point of the developer.
Other uses for microcapsules have been developed such as including pesticides within microcapsules as disclosed European patent application no. 0,064,379 published Nov. 10, 1982. The polymers used in forming the microcapsules which include the pesticides are indicated as having a glass transition temperature of from -15.degree. C. to 50.degree. C. Also in the agricultural field, capsules substantially larger than those of the present invention were used to encapsulate liquid fertilizers in U.S. Pat. No. 3,985,840, issued Oct. 12, 1976. Similar microcapsules are disclosed in U.S. Pat. No. 3,977,992, issued Aug. 31, 1976; see also U.S. Pat. No. 3,242,051, issued Mar. 22, 1966.
A completely different use for microcapsules is disclosed by Okahata, Y. in an article entitled "Lipid Bilayer-Corked Microcapsule Membranes--Reversible, signal-receptive permeation control" which was published in Acc. Chem. Res. (1986) vol. 19, pages 57-63. In this article attempts are made to produce microcapsules which mimic cell membranes in some respects. A related disclosure is made by Okahata in a letter to the editor published in "Macromolecules" (1986) vol. 19, pages 493-494.
Based on the above it is clear that there are a variety of different possible uses for microcapsules and that the microcapsules can be produced using a range of different types of materials using the same or different types of processing technology. A general overview of the field of microcapsules, their preparation, properties and potential applications is given by Wolfgang Sliwka in an article entitled "Microencapsulation" published in Angew. Chem., International Edition, Vol. 14 (1975) No. 8, pages 550-593. Based on this article it is clear that there are an infinite number of possible combinations of materials and processing technology which can be utilized to produce an infinite number of different types of microcapsules with different active ingredients therein. As explained in detail below the present invention is directed towards the use of particular types of polymeric materials, more particularly crystalline polymer materials (preferably side-chain crystallizable polymeric materials) which can form microcapsules which have heat-sensitive permeabilities and specific melting points.